Afterward, we’ll walk through the different C standard libraries that these two use. Next, we’ll understand service management, package management, and the default shell employed by the Docker images.
Alpine Linux and BusyBox originated for different use cases. In this section, we’ll dive into their histories and design philosophies to better grasp how each of them evolved and why they were created.
The origin of BusyBox dates back to the mid-1990s. In the beginning, it was designed to be used as a rescue disk as well as an installer for the Debian distribution. Due to its extremely compact design, users could fit it in a single floppy disk.
It was soon abandoned by the original author. However, there were forks of BusyBox, which were maintained for different projects like Debian and Linux Router Project. In 1999, the two forks were unified to build a performant and a small userland for embedded systems.
So, in essence, BusyBox is a suite of several Unix utilities merged into a single executable that runs in POSIX environments, like Linux containers, Android, and FreeBSD. However, it can also be customized to include more utilities. In addition, many of these tools solely interface with the Linux kernel.
Furthermore, the primary goal of BusyBox is to keep itself small. Therefore, the utilities it includes aren’t as comprehensive as their GNU counterparts. It also makes it more secure because there are fewer attack surfaces.
2.2. Alpine Linux
Alpine Linux is a Linux distribution that is usually used by embedded systems. Initially, it was based on Gentoo. Later on, it adopted the stripped-down version of the standard Linux kernel.
Alpine comes with BusyBox as its userland, a basic service manager, and a lightweight C standard library, thereby making it a suitable choice for containers, routers, virtual machines, as well as servers.
In the next sections, we’ll dig into more technical differences between BusyBox and Alpine Linux, as well as how their Docker images differ.
3. C Standard Library
BusyBox and Alpine Linux primarily differ in their default choice of C standard libraries. However, Docker provides images that include a set of different standard libraries that we can use with BusyBox and Alpine.
These C standard libraries provide crucial functions for programs written in C. These operations include I/O handling, memory allocations, networking, and other low-level operations.
In the next sections, we discuss how these images differ in terms of the C standard library.
3.1. Use of the C Standard Library
The older versions of the BusyBox image would statically link glibc (GNU C Library). In static linking, all of the libraries that a program depends on are combined into a single executable file at compile-time. Therefore, the program doesn’t rely on external libraries at runtime. In other words, the program is self-contained.
On the other hand, the recent Busybox image dynamically links glibc. In dynamic linking, all of the external code that a program requires isn’t included with the compiled executable. Instead, it uses shared libraries at runtime, which are loaded into memory when the program starts.
The prominent reason for this switch is the use of libnss (Name Service Switch) in the earlier Docker images. It was linked dynamically, even in the static build of BusyBox.
Furthermore, the official BusyBox Docker image uses glibc by default. However, there are several variants of the image, which provide the images for musl and uclibc as well.
The Alpine Docker image dynamically links against the musl. Before 2014, it used uclibc.
3.2. glibc vs. musl
glibc and musl conform to the C standard and attain the same goal, which is to provide standard functions to C programs. However, there are a few factors where they both differ.
Firstly, glibc is written for performance and portability. In addition, glibc also includes specific code improvements for performance, thereby increasing the codebase and its size. On the other hand, musl focuses on minimalism and code correctness. Therefore, it’s a bit slower than glibc at the expense of size reduction.
In addition, musl reports more meaningful error messages as opposed to glibc, which sometimes exits immediately in case of resource exhaustion.
Secondly, glibc is used by most C applications in the Linux ecosystem. On account of that, potential bugs are caught and patched early on. In contrast, new developers using musl encounter more quirks. The quirks aren’t necessarily related to musl itself but the way certain system calls or features are implemented in comparison to glibc.
BusyBox is licensed under GNU General Public License 2.0 (GPL). Similarly, glibc is licensed under the GNU Lesser General Public License (LGPL). For that reason, software licensed under GPL-compatible terms can be statically linked against it. On account of that, it creates more restrictions than musl.
Alpine Linux and musl are both licensed under MIT (Massachusetts Institute of Technology). So they’re usable with fewer restrictions.
4. Service Management
The Alpine Docker image uses the OpenRC (Run Command) service manager. It’s designed to be simple and efficient, providing basic init and process management. However, users have the freedom to incorporate other init systems if required.
In contrast, BusyBox includes a basic init system known as “init” or “init applet”. It’s more lightweight than OpenRC, making it more suitable for embedded systems. However, it lacks a lot of features as compared to other dedicated init systems.
5. Package Management
Alpine Linux, as well as the Alpine Docker image, comes with the APK (Alpine Package Keeper) package manager. It was first a collection of shell scripts, but later on, it was rewritten from scratch in C.
The primary goal of APK is to be more performant. It achieves this by writing new data in place into the file system instead of carrying out caching and compression beforehand.
In contrast, the BusyBox Docker image doesn’t include a package manager, as it’s merely a set of utilities.
By default, both the BusyBox and Alpine Docker images include the Almquist Shell (Ash). It’s a lightweight fork of Bourne shell that aims to be minimal and fast. This makes it suitable for resource constraint systems.
However, we can change the default shell to something else in Alpine Linux if we wish.
In this article, we discussed several differences between the BusyBox and Alpine Linux Docker images. We saw how both of them use different C standard libraries and the goals they achieve, which is to be fast and compact.
Furthermore, we also learned about the different technologies employed by them such as init systems, package managers, and shells.